Add a basic CONTRIBUTING.md (#117)

CONTRIBUTING.md

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+# Contributing
+
+SatchelJS welcomes contributions from the community.  By contributing, you agree to abide by the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct).
+
+## Problems and Questions
+
+If you encounter a problem with SatchelJS, please search in the [issue tracker]((https://github.com/Microsoft/satcheljs/issues)) to see if it has already been reported.  If not, create a new issue.
+
+## Development
+
+In order to contribute code to Satchel, you must first sign the [CLA](https://cla.opensource.microsoft.com/Microsoft/satcheljs), which can be found at https://cla.opensource.microsoft.com/Microsoft/satcheljs.  If you haven't signed it before submitting a Pull Request, you can access it through the `Details` link in the `license/cla` check.
+
+In order to submit a pull request:
+
+1. Fork and clone this repository
+2. Create a feature branch
+3. Make your changes and ensure they build: `yarn build`
+4. Add or update tests and ensure they pass: `yarn test`
+5. If necessary, update documentation in the `REAMDE.md`
+6. Create a [Pull Request](https://github.com/Microsoft/satcheljs/pulls)